Functional magnetic resonance imaging (fMRI) allows non- invasive spatial mapping of brain function in human subjects. Most previous fMRI studies of auditory processing have focussed on the cortex and have addressed cognitive, rather than basic neurophysiological issues. Our recent technical developments have allowed us to image, for the first time, sound-evoked activity in subcortical auditory structures, specifically the inferior colliculus and the medial geniculate body. Given the current spatial resolution of fMRI, we anticipate that other subcortical auditory nuclei will also be resolvable, including the cochlear nucleus and superior olivary complex. Thus, human auditory function can now be mapped from brainstem to cortex. Armed with this capability, we will examine the functional organization of the human auditory pathway. A wealth of animal data has provided considerable information about the organization of the mammalian central auditory system, but has also pointed out significant inter-species differences. Our fMRI experiments will establish whether fundamental components of the basic mammalian plan are applicable to the human. For example, we will examine the lateralization of function in individual subcortical structures (e.g., cochlear nucleus, superior olivary complex, inferior colliculus, medial geniculate body), and the frequency organization of different cortical areas. If predictions based on the animal work are contradicted, it will mean that current thinking about human auditory processing will need to be fundamentally revised. We will also use fMRI in subjects with peripheral high-frequency hearing loss to evaluate whether functional reorganization occurs at cortical levels, as has been demonstrated in the animal electrophysiology literature. The possibility that fundamental reorganization of auditory cortex occur subsequent to peripheral damage has profound implications for auditory rehabilitation via hearing aids or cochlear implants. The proposed experiments will provide information about the human auditory system which is obtainable in no other way. The results of these studies will constitute a significant advance in knowledge about both normal and abnormal central auditory function in humans.